Vaccinia virus has been developed into a novel eukaryotic expression vector with many uses. A chimeric gene is formed by ligating vaccinia virus transcriptional regulatory signals to a foreign protein coding sequence. Homologous recombination is then used to insert the chimeric gene into a non-essential region of the vaccinia virus genome. In a novel modification of the system, the bacteriophage T7 RNA polymerase gene was integrated into the vaccinia virus genome under control of a vaccinia promoter. The highly efficient and specific bacteriophage polymerase was then used to transcribe genes placed next to T7 promoters. In a further modification, the Escherichia coli lac operator system was used to regulate expression of the bacteriophage T7 RNA polymerase gene within the vaccinia virus genome. By doing this, it was possible for the first time to incorporate both the T7 RNA polymerase gene and a foreign gene under T7 promoter control in the same viral genome without compromising the viability of the virus. Expression of the foreign gene was obtained on addition of inducer. Still another modification of the system involved the formation of a cell line that constitutively express the bacteriophage T7 RNA polymerase thus eliminating the need to incorporate this gene into vaccinia virus. Several enhanced vectors were made for high expression with either the standard recombinant vaccinia system or the hybrid bacteriophage T7/vaccinia system. A new method of isolating recombinant vaccinia viruses, called reverse guanine phosphoribosyltransferase selection was developed. In collaborative studies, recombinant vaccinia viruses were used for expression of ion channels and receptors in frog oocytes, a candidate transmission blocking malaria vaccine, the poliovirus receptor, and varicella zoster envelope glycoproteins.